Even though the band thanked the "great COKE-cola company of Los Angeles" in the album's sleeve notes, it's been said by one or more band members that around the making of Vol 4 the booze and drugs stopped being fun and inner turmoil was starting to rise.
I noted in my review of Master of Reality, that despite the versatility of the material, the album manages to convey a sense of focus. That's not the case with this album, even though the majority of songs are of the familiar kind. The reason for this is mostly in the song Changes, which with its piano and mellotron is a big departure from their previous ballads that shared the dark somber mood of the heavier numbers. Of course it's not a happy song, though the opening lines "I feel unhappy / I feel so sad" may not be the most original depiction of sorrow.
But as mentioned, apart from a useless noise bit called FX and another beautiful instrumental Laguna Sunrise, most of the album is dedicated again to the heavier pounding. Tomorrow's Dream, Supernaut and Snowblind are true classics, and Cornucopia has some of Sabbath's heaviest riffs and some tricky rhythms, but the lighter rocker St. Vitus Dance might be early Sab at their most expendable. Opening number Wheels of Confusion on the other hand is an underrated piece of work with a great instrumental section and one extraordinarily powerful riff towards the end just before Ozzy comes back to the mic.
The closer Under the Sun would benefit from better production, as the slow opening riffs are tailor made for good thundering assault, but the sound of the album is a slight step back from the previous production, guitars ringing a bit too hollow to let the heavy riffs shine like they should. Hearing the studio version of Cornucopia for the first time was a great disappointment, as I'd heard the song first as the massive live rendition found on Live at Last/Past Lives. Just like St. Vitus Dance, Under the Sun also has its share of weak riffs, but is somewhat redeemed by a great, long outro also serving as a platform for multi-tracked soloing.
A let down after two masterpieces, but nevertheless an excellent album with many classic moments.